The goal of this project is to determine whether human neutrophil (PMN) surface antigen expression is altered after PMN have migrated towards an inflammatory stimulus. We have previously shown that PMN adhesion molecule expression is differentially downregulated after PMN chemotaxis. The present study was undertaken to examine the changes that occur in PMN surface molecule expression of aminopeptidase N (CD13) and the Fcg receptors, FcRII (CD32) and FcRIII (CD16), after PMN chemotaxis in vitro. Two populations of PMN are easily identified and separated with an in vitro polycarbonate membrane chemotaxis system; one population does not migrate towards chemoattractant (nonmigrating population) and remains on the upper surface of the membrane, whereas the other population migrates through the membrane pores to the lower surface of the membrane (migrating population). PMN, incubated in suspension with or without the N-formyl peptide chemoattractant (FMLP), were compared with the migrating and nonmigrating subpopulations which were exposed to a gradient of FMLP. PMN were stained with a panel of monoclonal antibodies which recognize aminopeptidase N (CD13), or the Fcg receptors, FcRII (CD32) and FcRIII (CD16), and surface expression was quantified with a flow cytometer. CD13 expression was not significantly altered after PMN chemotaxis. CD13 expression on migrating and non-migrating PMN populations was equivalent. In contrast, FcRII and FcRIII expression was down- regulated on the migrating PMN subpopulation by 42% and 58%, respectively, when compared with the non- migrating PMN subpopulation. Down-regulation of CD32 did not occur when non-adherent PMN were stimulated with chemoattractant in suspension, however, CD16 expression was down-regulated by 42% when non-adherent PMN were stimulated with FMLP in suspension. The results of this study demonstrate that PMN surface molecule expression is selectively down-regulated after PMN chemotaxis. This experimental system provides a means for quantifying the expression of myeloid antigens which are important in inflammatory responses.